Lignocellulosic biomass, also referred to as biomass, includes waste materials such as corn stover, sawdust, straws, bagasse, municipal solid waste (paper and cardboard), and dedicated lignocellulose crops such as poplar, miscanthus, and switchgrass. These agricultural and waste materials are being developed as alternate domestic sources for production of carbon-based products such as fuels, chemicals, and the like. Lignocellulosic biomass is an attractive feedstock because it is an abundant, domestic, renewable source that can be converted to carbon-based chemicals or liquid transportation fuels.
Terrestrial biomass (lignocellulosic material) is composed of three major components: cellulose (30-50%), a highly crystalline polymer of cellobiose (a glucose dimer); hemicellulose (15-30%), a complex amorphous polymer of five-(pentose) and six-(hexose) carbon sugars; and lignin (5-30%), a highly cross-linked amorphous polymer of phenolic compounds. Lignin is a polyphenyl propanoid macromolecular assembly that is covalently cross-linked to hemicellulose. These components of biomass can serve as a source of carbon-based feedstock for fuel and chemical production in much the same way that crude oil serves as the carbon feedstock in petrochemical refineries. Cellulose and hemicellulose are the major polysaccharide components which, when hydrolyzed into their sugars, can be converted into ethanol or butanol fuel, polymer precursors such as 1,3-propanediol, lactic acid, or other products through various fermentation methods. These sugars also form the feedstock for production of a variety of chemicals and polymers through chemical conversion or fermentation. However, the complex and compact structure of lignocellulosic biomass renders this feedstock largely impenetrable to water, catalysts, or enzymes used to hydrolyze its constituent polysaccharides to monomeric sugars (saccharification).
In its natural state, cellulose is highly crystalline in structure with individual cellulose polymer chains held together by a strong hydrogen bonding network and van der Waals forces. The individual cellulose chains are linear condensation polymer molecules of anhydroglucose units covalently linked by β-1,4 glycosidic bonds with degrees of polymerization (dp) ranging from, typically, 1,000 to 15,000 units. The high crystallinity of cellulose, while imparting structural integrity and mechanical strength to the material, renders it recalcitrant towards hydrolysis aimed at producing glucose (which is a feedstock for producing fuels and chemicals) from this polysaccharide.
In lignocellulosic biomass, crystalline cellulose fibrils are embedded in a less well-organized hemicellulose matrix which, in turn, is linked to lignin. Hydrolysis of cellulose and hemicellulose polysaccharides into their monomeric sugars, glucose, xylose, and other sugars, provides the basic precursors useful for producing fuels (i.e., ethanol or butanol) and chemicals from biomass via the sugar platform. However, biomass is not easily penetrated by water or enzymes and must be pretreated to realize high yields of sugars during enzymatic hydrolysis of the polysaccharides. Enzyme hydrolysis is often favored over mineral acid-catalyzed hydrolysis since mineral acids produce sugar degradation products such as hydroxymethyl furfural (HMF), furfural, levulinic acid, and formic acid. These sugar degradation products are inhibitory to downstream fermentation steps.
Due to the structural complexity of lignocellulosic biomass and the inaccessibility of biomass polysaccharides to water and catalysts, hydrolysis rates to the monomeric sugars that form the sugar platform are extremely slow. Proper pretreatment of the biomass is therefore required to enable efficient saccharification of the cellulose and hemicellulose components to their constituent sugars. The pretreatment generally required for biomass is more severe than that required for starch-based (i.e. corn grain) ethanol production. The hydrolysis of biomass polysaccharides (cellulose and hemicellulose) also requires a complex mixture of enzymes (cellulases and hemicellulases) due to the heterogeneity of hemicellulose and the crystalline nature of cellulose in contrast to the less complex amorphous structure of starch-based feedstocks (amylose). Effective pretreatment and hydrolysis (saccharification) of biomass present key challenges in the development of sustainable processes for chemical and fuel production from biomass. Current pretreatment approaches suffer from slow reaction rates of cellulose and hemicellulose hydrolysis (using cellulases and hemicellulases), low sugar yields, and degradation of biomass or pretreatment chemicals due to the severity of the current pretreatment processes.
Furthermore, enzymatic access to cellulose, for hydrolysis, is restricted by hemicellulose and lignin. Neither the water molecules nor the catalysts for hydrolysis (saccharification) are able to easily penetrate the crystalline matrix of cellulose. As a remedy, slow reaction rates have been increased by pretreatment involving an ionic liquid incubation of the biomass, which is capable of partially dissolving the cellulosic and hemicellulosic portion at various temperatures ranging between about 120° C. and about 160° C., and resulting in higher digestibility yields. However, the high temperature incubation is not favorable from an energy input standpoint. High temperatures can also lead to degradation of the feedstock and ionic liquid, as well as the promotion of unwanted side reactions. Thus, there remains a need for efficient methods of enhancing saccharification of cellulose and hemicellulose from biomass for fuel and chemical production that do not require high temperatures.